The present invention relates to exhaust processors for treating emissions from combustion product produced by an engine, and particularly to an apparatus for rapidly heating a catalytic converter or other exhaust processor to its minimum operating temperature at the beginning of a cold start cycle of an engine. More particularly, this invention relates to an exhaust processor including a catalyzed substrate and a substrate housing configured to use hot combustion product to heat the catalyzed substrate quickly.
For environmental reasons, engine exhaust must be cleaned on board a vehicle before it is expelled into the atmosphere. This processing is accomplished by passing the untreated combustion product produced by the engine through an exhaust processor to minimize unwanted emissions.
Catalytic converters are well-known exhaust processors and are used to purify contaminants from hot combustion product discharged from an engine exhaust manifold. Within a catalyzed exhaust processor, the combustion product is treated by a catalyzed ceramic or metal substrate which converts the exhaust gases discharged from the engine primarily into carbon dioxide, nitrogen, and water vapor. The catalytic converter treats engine combustion product to produce an exhaust stream meeting stringent state and federal environmental regulations and emissions standards. After processing, the treated combustion product is then routed to a muffler to attenuate the noise associated with the combustion. It is also known to provide exhaust processors that include substrates that function as particulate traps to filter contaminant particulates without using a catalyst.
Exhaust processors are known in the prior art. See, for example, U.S. Pat. No. 4,969,264 to Dryer et al.; U.S. Pat. No. 3,159,239 to Andrews; U.S. Pat. No. 4,087,039 to Balluff; U.S. Pat. No. 4,519,120 to Nonnenmann et al.; and European patent No. 0 243 951 to Kanniainen.
Typically, hot combustion product is conducted through a pipe mounted under the body of a vehicle between an engine and a remote exhaust processor. The temperature of the combustion product decreases somewhat during this journey. At the beginning of a cold start cycle of an engine, the exhaust processor is "cold" and typically has a temperature that is about equal to the temperature of the surroundings. Over time, the combustion product produced by a cold-started engine, being at an elevated temperature, heats the substrate and housing in the exhaust processor to a high temperature. This heating is desirable if the substrate is catalyzed because a catalyzed substrate works to purify contaminants from engine combustion product most efficiently at high temperatures.
A catalyzed substrate purifies contaminants from engine combustion product most efficiently at high temperatures. However, a catalyzed substrate does not actively and efficiently treat combustion product until it is heated to a minimum operating temperature during the initial moments of an engine cold start cycle. A catalytic converter is said to "light off" when it is heated to its minimum operating temperature and begins to purify combustion product in an effective manner.
A substantial reduction in tail pipe emissions measured using the Federal Test Procedure can be realized by minimizing the elapsed time between engine ignition and catalytic converter light off during an engine cold start cycle. The majority of total emissions occurs during the cold start portion of the Federal Test Procedure before the catalytic converter has been heated to reach its minimum operating temperature. Accordingly, vehicle emissions can be reduced by achieving faster light off of the catalytic converter at the beginning of an engine cold start cycle.
With respect to the above-noted problem, U.S. Pat. No. 4,731,993 to Ito et al discloses a rear exhaust manifold having thick walls and a front exhaust manifold made of a thin stainless steel plate so that the front exhaust manifold has walls thinner than the walls of the rear exhaust manifold. It is also known from U.S. Pat. No. 5,018,66 to Cyb to apply a thin layer of heat-resistant compound to the interior of an exhaust manifold and from U.S. Pat. No. 5,004,018 to Bainbridge to provide an insulated exhaust pipe including inner and outer spaced tubes separated by refractory fiber insulation. Systems using electrically heated catalytic converters and catalytic converters containing increased amounts of precious metals are also known.
There is a need to improve vehicle emission controls to meet increasingly stringent emission standards. An exhaust system configured to provide quicker light off of the catalytic converter using heat energy contained in the hot combustion product produced by an engine would be an improvement over conventional exhaust systems.
Conventional exhaust processors typically use either heavy gauge metal clamshells welded together or a heavy gauge metal can with heavy gauge metal cones welded to each end to provide housings supporting catalyzed substrates. Because of the heavy gauge metal structure, conventional substrate housings and support structures have a high "thermal capacitance". That is, the heat energy storage capability of these conventional housings and structures per unit length is quite large and they act as large heat sinks during the initial moments of an engine cold start cycle.
As a result of the high thermal capacitance of the conventional substrate housings and support structures, a large portion of the heat energy from the combustion product is consumed in heating the heavy gage substrate housings and support structures. By allowing heat energy from the combustion product to be diverted to the substrate housing and support structure, less heat energy is available to heat the substrate to its minimum operating temperature. Consequently, it takes longer to heat the catalyzed substrate to its minimum operating temperature at the beginning of a cold start cycle of an engine.
It would therefore be desirable to reduce the amount of heat energy used to heat a substrate housing and support structure during the initial moments of an engine cold start cycle to raise the temperature of the substrate to reach its minimum operating temperature in less time. Tail pipe emissions would be reduced if the substrate in an improved exhaust processor reached its minimum operating temperature at an earlier point during an engine cold start cycle.
Conventional exhaust processors are known to radiate large amounts of heat to the area surrounding the exhaust processor. Various shielding designs are typically used to protect objects in the surrounding area from the heat generated by the exhaust processor. Generally, conventional exhaust processor shields include flanges at a clamshell split line to permit the shields to be attached to each other and surround the exhaust processor. However, the flanges cause a processor location problem because it is necessary to provide a larger clearance envelope around the processor to accommodate large flanges. Therefore shielding or insulating the processor without significantly increasing the size of the processor would be an improvement over conventional exhaust processors.
According to the present invention, an exhaust processor assembly comprises an outer shell formed to include an interior region and an inner shell extending into the interior region. The exhaust processor assembly includes substrate means for treating emissions contained in combustion product emitted from an engine. The inner shell includes means for positioning the substrate means inside the interior region of the outer shell so that the substrate means is positioned in spaced-apart relation to the outer shell to minimize thermal transfer between the substrate means and the outer shell.
In preferred embodiments, the positioning means includes a thin-walled sleeve and the substrate means is retained in this thin-walled sleeve to lie in spaced-apart relation to the outer shell. The thin-walled sleeve desirably has a low thermal capacitance of less than 12,200 ##EQU1## so it does not act as a significant heat sink to divert heat energy in the combustion product away from the substrate means at the beginning of an engine cold start cycle. Also, the thin-walled sleeve positions the substrate means in spaced-relation to the outer shell to minimize diversion of heat energy in the combustion product to the more massive outer shell. Advantageously, the outer shell is configured to protect and support the thin-walled sleeve and substrate means without absorbing a lot of heat from combustion product at engine start up.
By providing an outer shell for structural strength, the present invention allows the use of a thin-walled inner shell. This low thermal capacitance thin-walled inner shell provides an improvement over conventional exhaust processors in that it causes the substrate in the exhaust processor to be heated to its minimum operating temperature and light off more rapidly at the beginning of a cold start cycle of the engine. Consequently, the substrate is active to lower total vehicle emissions without resorting to complex exhaust control mechanisms, costly exhaust system materials, or electrically preheated substrates. Essentially, the low thermal capacitance thin-walled inner shell conserves the heat energy already available in the hot combustion product discharged by the engine and uses that heat energy to effectively light off the substrate very early in the cold start cycle of an engine and reduce total emissions and resulting pollution.
The present invention represents another improvement over conventional processors by providing an insulated exhaust processor. The present invention positions an insulating air gap between the inner and outer housing which obviates the need for shielding, thereby allowing a smaller clearance envelope while actually reducing the amount of heat given off by the exhaust processor.
Additional objects, features, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived.